1. Field of the Invention
The present invention relates to ink-jet printing apparatus for printing by ejecting ink droplets onto print media, and more particularly to an ink-jet printing apparatus which is capable of providing stable performance of ink droplet ejection by driving a printing head thereof with a drive voltage varying with ink viscosity.
2. Description of Related Art
Among conventional ink-jet printing apparatuses, there is a drop-on-demand arrangement of a shear mode type using piezoelectric ceramic material as disclosed in Japanese Unexamined Patent Publication No. 63-247051. A printing head used in this kind of ink-jet printing apparatus is shown in FIGS. 12 and 13.
Referring to FIG. 12, a printing head 21 is provided with a cover plate 201 and a base plate 202 which is provided facing the cover plate 201. A part between the cover plate 201 and the base plate 202 is formed with piezoelectric material so that partitions are provided by a plurality of shear mode actuator walls 203 polarized in arrow directions F30 and F40 indicated in FIG. 12, and an ink channel 205 and an air channel 212 are arranged alternately between each pair of shear mode actuator walls 203. One side of each shear mode actuator wall 203 has a film electrode 204, and the other side thereof has a film electrode 214.
As shown in FIG. 13, the front end of the shear mode actuator wall 203 is provided with a nozzle plate 207 which has nozzles 206 each of which is connected with the ink channel 205, and the rear end of the shear mode actuator wall 203 is provided with a manifold part 209 which has a filler part 208 for preventing intrusion of ink from a common ink passage 213 into the air channel 212. The manifold part 209 is used for distributing ink from an ink reservoir (not shown) to each ink channel 205. Each of the electrodes 204 and 214 is covered with an insulating layer (not shown), and the electrode 214 facing the air channel 212 is connected with the ground 211. The electrode 204 forming the ink channel 205 is connected with a head driver IC (integrated circuit) 83 which applies an actuator drive signal to the electrodes 204 and 214.
In the printing head 21 structured as mentioned above, when the head driver IC 83 applies the actuator drive signal to the electrode 204, piezoelectric thickness slip deformation occurs on each shear mode actuator wall 203 to increase a volume of the ink channel 205. For instance, as shown in FIG. 14, when a positive drive voltage is applied to the electrode 204 of the ink channel 205, electric fields are produced on the shear mode actuator wall 203 in arrow directions F10 and F20, causing piezoelectric thickness slip deformation to occur on upper walls 203a and lower walls 203b of the shear mode actuator wall 203 so that the volume of the ink channel 205 is increased. At this step of operation, pressure in the ink channel 205 including a vicinal part of the nozzle 206 is decreased. This state is maintained during a period of one-way propagation time T of a pressure wave in the ink channel 205, thereby letting ink be supplied from the common ink passage 213 thereinto.
The one-way propagation time T indicates a period of time required for a pressure wave in the ink channel 205 to complete propagation in the longitudinal direction of the ink channel 205. Using length `L` (FIG. 13) of the ink channel 205 in the longitudinal direction thereof and acoustic velocity `a` in ink in the ink channel 205, `T` is expressed as follows; T=L/a.
Based on the principle of pressure wave propagation, pressure in the ink channel 205 is reversed to become positive after a lapse of time T following application of the drive voltage. At the timing of pressure reversal, the drive voltage being applied to the electrode 204 of the ink channel 205 is reset to zero (0) V. Thus, the shear mode actuator wall 203 is restored to normal (FIGS. 12, 13), applying pressure to ink. At this step of operation, the positive pressure is added to pressure which has been produced by restoration of the shear mode actuator wall 203 to normal, so that relatively high pressure is generated in the vicinity of the nozzle 206 in the ink channel 205, thereby ejecting ink from the ink channel 205 to the outside through the nozzle 206.
In such a conventional ink-jet printing apparatus as mentioned above, pressure wave oscillation remains in an ink channel even after ink is ejected. Particularly, when viscosity of ink is low, residual pressure wave oscillation may cause undesired ink droplets to be sprayed or ejected accidentally through the nozzle 206. This unstable ejection of ink droplets makes it rather difficult to attain satisfactory quality of printing.
In another conventional arrangement, disclosed in Japanese Unexamined Patent Publication No. 62-299343 for example, a print pulse for ink droplet ejection is followed by a cancel pulse to reduce residual pressure wave oscillation in an ink channel. More specifically, although a pressure wave for ink droplet ejection rebounds from the front and rear ends of the ink channel and a nozzle meniscus is vibrated after a lapse of time 4T following the start of ink droplet ejection, a pressure wave for phase reversal is produced to cancel this phenomenon.
However, in such an arrangement that the cancel pulse is generated after a lapse of time 4T following the start of ink droplet ejection, a negative power supply for generating reverse-phase cancel pulses is required in addition to a positive power supply for generating ink emission pulses, causing disadvantages of complexity in an electronic control circuit and an increase in production cost.
A further conventional arrangement is disclosed in Japanese Examined Patent Publication No. 6-9920. In this arrangement, a voltage setup part comprising four on-off contacts connected in parallel is provided at an interface of a printing head so that one of 16 drive voltage levels can be selected, and one of these contacts is turned on/off according to characteristics of the printing head for the purpose of setting up an optimum drive voltage. However, this arrangement fails to consider variations in the viscosity of ink caused by variations in the ambient temperature.
More specifically, viscosity of ink used in an ink-jet printing apparatus varies with ambient temperature as shown in FIG. 15, which shows a relationship between viscosity of ink and ambient temperature thereof. For instance, the viscosity of ink is approximately 3 mpa.s at an ambient temperature level of 25.degree. C., but it becomes approximately 6 mpa.s at 10.degree. C. and approximately 2 mpa.s at 40.degree. C. As the viscosity of ink varies thus, a volume of ink per droplet still varies to result in variations in printing quality as shown in FIGS. 16A to 16D.
That is, as shown in FIG. 16A, when a droplet of ink 293 is ejected through the nozzle 206 formed on the nozzle plate 207 in the same manner as shown in FIGS. 12 and 13, a meniscus 292 protrudes from the nozzle 206 frontward (in ejecting direction) due to positive pressure in residual pressure variation in the ink channel 205. Then, as shown in FIG. 16B, the meniscus 292 retracts from the nozzle 206 backward (toward the ink channel 205) due to negative pressure. In this fashion, the meniscus 292 oscillates, and oscillation thereof ceases after a lapse of a certain period of time.
In an experiment, the following phenomena were ascertained. As viscosity of ink decreases, oscillation of the meniscus 292 increases to prolong a period taken until oscillation ceases. Therefore, if the drive voltage is applied to the shear mode actuator to produce pressure in the ink channel 205 through piezoelectric thickness slip deformation before oscillation ceases, a rate of ink droplet ejection increases and also a volume of ink per droplet increases.
Since surface tension of the meniscus 292 on an opening plane of the nozzle 206 decreases with a decrease in viscosity of ink, the meniscus 292 droops to hang out to a surface of the nozzle plate 207 as shown in FIG. 16C. In the presence of a hanging part 294 of the meniscus 292, the subsequent droplet of ink 293 is ejected in a deviated direction as shown in FIG. 16D.
As the viscosity of ink decreases to increase the rate of ink droplet ejection, the volume of ink per droplet causes deviation of the ink droplet ejecting direction, resulting in degradation in performance of ink droplet ejection.